Air-fuel ratio cylinder imbalance is a condition in which the air-fuel ratio in one or more cylinders is different than the other cylinders. Air-fuel ratio cylinder imbalance may occur as a result of a cylinder specific malfunction, such as an intake manifold leak at a particular cylinder, a fuel injector malfunction, an individual cylinder exhaust gas recirculation (EGR) valve runner flow delivery abnormality, an individual variable cam life malfunction and/or erroneous cam lift profile, or other malfunctions that will cause improper distribution of intake air or fuel to the engine.
When air-fuel ratio cylinder imbalance occurs in one or more cylinders, the fuel delivery system is unable to maintain a proper vehicle emission level. In addition, vehicle data has shown that the impact of tailpipe emissions due to air-fuel ratio cylinder imbalance varies depending on whether there is an air-fuel ratio cylinder rich imbalance or an air-fuel ratio cylinder lean imbalance. Vehicle on-board diagnostics are required to detect air-fuel ratio cylinder imbalance levels that cause the fuel delivery system to become unable to maintain vehicle emissions at or below 1.5 times any of the applicable Federal Test Procedure (FTP) standards.
Existing systems use intrusive monitoring to differentiate between rich and lean cylinder imbalance malfunction. Fueling is shifted for each cylinder individually to determine whether the cylinder(s) is running rich or lean during the monitoring event. As such, tailpipe emissions increase during monitoring. Any failure event is typically counted by a fail counter and a de-bounce counter.
Government regulations and consumer desires demand that vehicles continuously improve on fuel economy and emissions. At the same time, vehicle affordability is a concern, in light of the numerous automotive requirements and increasing costs. Accordingly, there is a need for more fuel efficient and low-emission engines that are also without added complexity and cost.